Can't get much help on other forums

[Paul Birkeland]

Wow, that's a lot of hum!

Shorting the input wires together should be good enough, shorting them together and then touching the chassis is even better.

The next test is to measure the AC voltage between the output of C3 and ground, as well as C4 and ground.  The output side will have 0V DC on it.  If both sides of one or both caps have DC on them, then replace that cap.

-PB

[Doc B.]

Remember that he's measuring that hum with the output unloaded. Not saying it isn't very high, but it might be worth putting an 8 ohm output load resistor across the output when making the measurement to get a more realistic assessment of the hum level with a speaker load attached.

[rockpassion]

Doc & Paul,

I made this measurement with an 8 ohm speaker attached.  Thanks for the info.  I will do the other tests and get back.  Does it matter which side of the capacitor I am measuring?  Am I grounding the black lead to chassis or some other ground within the circuit?

Richard

[Paul Birkeland]

Now that you know that the power supply, chassis, and incoming earth are all connected, you have some flexibility in terms of where you ground your probe.  For the AC and DC readings, any of these will work. 

I would recommend measuring the AC on the side of the cap with no DC voltage present.

-PB

[rockpassion]

Paul,

I made the measurements you suggested.  I grounded the black lead and took my measurements.  On C3 on the side going to V1 I get a reading that starts at 21.83 and drops and seems to stabilize at about 9.65 VDC.  On the V3 connection I get -6.3 dropping to -4.5 VDC.  I took the reading on the V3 connection for AC and got 85 VAC.  On C4 on the side going to V2 it started at 19.05 VDC and dropped to 9.98 VDC and on the V3 connection it started at -6.4 VDC dropping to -3.92 VDC.  Again, I checked the VAC on the V3 connection and got 89.3 VAC.

Since voltage is measured relative to ground does the negative voltage indicate bad capacitors?  I do have a set of spares for these so I can replace them if need be.

Richard

[Paul Birkeland]

You are saying "V" and numbers, are you measuring the caps in question? 

These should show somewhat high voltage on one side, and something close to 0V on the other.

Some negative voltage would indicate either the existence of a negative voltage supply, or excessive grid leakage current from the next stage would make negative voltage.

Positive voltage would most likely be leakage through the cap itself.

[rockpassion]

Paul,

Sorry, I did measure C3 and C4, the V1 & V2 refer to the 6SL7's and V3 refers to 6SN7.  The connections refer to the wires going to these tubes.  I hope that helps. 

So, it sounds like the negative voltage is not indicating a bad capacitor but something else?

Richard

[Paul Birkeland]

On C3, the side that goes to V1 needs to be WAY higher than 9V.  Without spending a lot of time modeling the circuit, there should be 100+V there.

On the V3 side, looking more carefully, there should be some bias voltage on that grid.  You should see the same voltage on pin 1 and pin 4 of the 6SN7 socket.

In any event, if you have old 0.01uF caps and old 1M carbon comp resistors, spend $4 and remove them and toss them.  For that matter, replacing R14 and R15 with decent metal film resistors should go a long way towards having a reliable amp.

For me, however, the next thing I would want to do is to start looking at the flow of B+, because it doesn't seem to be getting through to your driver stage.  R6 and R9 are your plate load resistors for the 6SL7, what do you see for DC voltages where they connect together?  (Also replace those if they are carbon comp resistors)

-PB

[Doc B.]

Not to sound like some old know it all fart, but if someone manages to talk me into restoring an older piece of gear for them (you're probably going to need to be female and rather charming), I just chuck all of the caps and replace them with modern equivalents. Ditto any carbon comp resistors. The amount of time spent wanking around with bad voltages and noise is cut down by about 90%.

[rockpassion]

Doc,

After the previous post by Paul it dawned on me to do exactly that.  I have already replaced all the Caps and the two diodes. So, I might as well spring for all the resistors they aren't that much.  So this weekend I will order all the resistors and install them before I continue any further. 

Thanks for the feedback, being frugal has its place but not in this case.  Resistors are cheap and I might as well replace them with newer metal film resistors.  This is a real learning experience.

Richard

[rockpassion]

Paul,

You may not hear from me for awhile.  I took Doc's advice and ordered replacements for all the resistors.  As soon as I get them I will replace them and cleanup and resolder all the wire connections. 

I do have one additional question but I will have to take a couple of pictures to explain myself.  I will try to get those early next week. 

Once again thanks for all the help.

Richard